With the recent advancement of the industry and technology, there is an ever-increasing demand for the development of a coating material equipped with sufficient heat resistance, chemical resistance and corrosion resistance to permit its use even in an ultimate environment. Poly(p-phenylene thioether) (hereinafter abbreviated as "PPTE"), a kind of poly(arylene thioether), has conventionally been used as a coating material by making use of its characteristic properties such as high corrosion resistance, high chemical resistance and good adhesion properties. However, PPTE has a melting point not higher than about 285.degree. C. and is still dissatisfactory in heat resistance for applications such that it is used for a long period of time in a high-temperature environment of 300.degree. C. or higher for instance.
Polyether ether ketone (hereinafter abbreviated as "PEEK") and polyether ketone (hereinafter abbreviated as "PEK") have recently been developed as heat-resistant resins. These resins have melting points of about 300.degree. C. or higher and moreover, they are crystalline thermoplastic resins.
PEEK and PEK however use expensive fluorine-substituted aromatic compounds as their raw materials. They hence involve such problems that they are costly and their use is limited for economical reasons.
Based on an assumption that PTK could be a promising candidate for heat-resistant thermoplastic resin like PEEK and PEK owing to their similarity in chemical structure, PTK has been studied to some extents to date.
There are some disclosure on PTKs, for example, in German Offenlegungsschrift 34 05 523Al (hereinafter abbreviated as "Publication A"), Japanese Pat. Laid-Open No. 58435/1985 (hereinafter abbreviated as "Publication B"), Japanese Pat. Laid-Open No. 104126/1985 (hereinafter abbreviated as "Publication C"), Japanese Pat. Laid-Open No. 13347/1972 hereinafter abbreviated as "Publication D"), Indian J. Chem., 21A, 501-502 (May, 1982) (hereinafter abbreviated as "Publication E"), Japanese Pat. Laid-Open No. 221229/1986 (hereinafter abbreviated as "Publication F"), and U.S. Pat. No. 4,716,212 (hereinafter abbreviated as "Publication G").
Regarding the PTKs described in the above publications, neither molding nor forming has however succeeded to date in accordance with conventional melt processing techniques. Incidentally, the term "conventional melt processing techniques" as used herein means usual melt processing techniques for thermoplastic resins, such as extrusion, injection molding and melt spinning.
The unsuccessful molding or forming of PTKs by conventional melt processing techniques is believed to be attributed to the poor melt stability of the prior art PTKs, which tended to lose their crystallinity or to undergo crosslinking and/or carbonization, resulting in a rapid increase in melt viscosity, upon their melt processing.
It was attempted to produce some molded or formed products in Publications A and B. Since these PTKs had poor melt-stability, certain specified types of molded or formed products were only obtained by a special molding or forming process, where PTKs were used only as a sort of binder, being impregnated into a great deal of reinforcing fibers of main structural materials and molded or formed under pressure.
Since conventional PTKs obtained by any of the production processes disclosed to date had low melt-stability, they tended to undergo decomposition, foaming, uneven curing or the like when employed to coat a metal base so as to form a fusion-coated layer thereon. It was hence difficult to form on a base material a fusion-coated uniform layer containing little defects such as pinholes and having high smoothness and sufficient peeling strength.